Isocyanates can enter the body through the respiratory tract or the skin.
Employees in factories where isocyanates are produced or processed are directly at risk. Also at risk are the workers who process certain products containing isocyanates, which react to form polyurethanes only during use (e.g. one- or two-component PUR adhesives, varnishes or assembly foams). This may cause high isocyanate emissions. The so-called monomeric isocyanates – i.e. isocyanates that have not been polymerised and are therefore still reactive molecules (see excursus*) – are of great relevance to health; although monomers are usually only present in small quantities in products containing isocyanates, they are very volatile and can thus be easily inhaled.
Once isocyanates have been converted (i.e. all isocyanate groups have been used up and chemically transformed) – e.g. in the case of cured PUR paints or wood-based panels containing isocyanates – they generally no longer pose a health hazard. However, isocyanates can be released unintentionally: e.g. during the thermal decomposition of plastics and during the storage or mechanical processing of incompletely converted PUR products.
Effects on humans and the environment
Isocyanates have a strong irritating effect on eyes, skin and mucous membranes – even in low concentrations. In higher doses, they cause, among other things, severe coughing and shortness of breath. In addition, isocyanates are highly allergenic: they cause sensitisation of the skin and respiratory tract (this is also referred to as isocyanate asthma). The allergising effect can already occur at concentrations far below the workplace limits.
Some isocyanates are considered to be possibly carcinogenic.
The gas phosgene, which is needed for the production of isocyanates, is highly toxic and hazardous to water (phosgene was used as a chemical warfare agent in World War I).
In 1984, isocyanates – or more precisely methyl isocyanate – were the cause of one of the biggest chemical disasters in history: In Bhopal, India, a tank containing the chemical exploded in a pesticide factory, causing tens of thousands of deaths and hundreds of thousands of injuries.
* Excursus: Monomeric isocyanates are single molecules with reactive isocyanate groups – as monomers they are a building block for the construction of larger molecules, the polymers. Oligomers are medium-sized molecules that are composed of several monomeric isocyanates or can also represent short chains of the polymer. Short chains of the polymer with reactive isocyanate groups are also called prepolymers. These are prepolymerised raw materials which – in the case of polyurethane – are produced by the reaction of isocyanates with polyols (special alcohols). In products containing isocyanates, isocyanates are usually present as prepolymers or as oligomers of several monomeric isocyanates, and as monomers only in small proportions. In contrast to monomers, oligomers and prepolymers are less volatile.